I have debian9 on my laptop, when open firefox I have this message "server not found" laptop connect to the internet and having signal but fierfox can not open any page,I try to use ping and router ip the msg destination host unreachable ?!! and network unreachable ! but the router name is found and connected to it ,when show setting of router in debian the signal is strong.

Like you do here, we need to see how you actually ran the command, and what it really says. It is all ways a good idea to look at the manual, when some one tells you use this or that command. For example, here, when acewiza told you use :

If you had sudo installed, it would have run a command as root, and you didnot even know for sure what the command does. We all mean well, and I know "acewiza" would not intentionally tell you to type a command that would harmyour system, neither would I, how ever some times the person trying to helpmay misunderstand, and tell you to use a option, that does something differentthe what you really wanted to do. You should all ways look up the command, using the 'man' command. Look at the various options, and make sure the command ,option , is going to do what you really want:

$man ifconfig----snip---- -a Causes ifconfig to print information on all interfaces. The protocol family may be specified as well. This is the default, if no parameters are given to ifconfig.

In this case, the -a option is harmless, and does what is expected. Looking up the command in the manual also is essential in order to fullyunderstand what it does, and will help you understand the solutions peoplegive when trouble shooting your system.

by acewiza » If you cannot do that, you may not be qualified to even troubleshoot the problem.

Do you know how to capture, or put the output into a text file, when you are using the console, or a terminal ? We may need to explain that as well, beforewe can start helping you. Let us know, if we need to do that first.

We also need some more info, is this a fresh install of Debian 9 ? orwas it working fine, and some thing changed, and now you no longer can connect. I have some chores to do , so have to go now. Apology on saying you needed to be root to run the command, I thoughtyou did, but when I tried it as a normal user, I saw that root (su) is not needed,That is another thing, you should all ways see if a command will work as a normal user, first, and only use 'su' if needed.

@GarryRicketson thank you so much for all this , I'm now discover that I have problem in Wi-fi, when became very near from router I get very good internet connection, but when became far from it there is no internet connection , Why how can get rid of this problem